1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of marine seismic data acquisition.
2. Description of the Related Art
In the field of geophysical prospecting, knowledge of the subsurface structure of the earth is useful for finding and extracting valuable mineral resources, such as oil and natural gas. A well-known tool in geophysical prospecting for seismic data acquisition is a seismic survey. In a seismic survey, seismic waves are generated by appropriate seismic sources and transmitted into the earth. In a land-based seismic survey, the seismic signal is generated by injecting a seismic signal from on or near the earth's surface, which then travels downwardly into the subterranean formations located in the subsurface of the earth. In a marine seismic survey, the seismic signal also travels through a body of water before reaching the subsurface of the earth.
When the seismic signal encounters a seismic reflector, such as an interface between two different rock formations having different acoustic impedances, a portion of the seismic signal is reflected back toward the surface. The seismic signals reflected from the subterranean formations are then detected by appropriate seismic sensors. The sensors measure amplitudes of physical aspects of the passing seismic signals, convert the measurements to electrical signals, and transmit those signals to an appropriate location for storage and processing. Seismic data processing techniques are applied to the collected seismic data to estimate the structure and possible hydrocarbon content of a portion of the earth's subterranean formations.
The appropriate types of seismic sensors typically include particle velocity sensors, particularly in land surveys, and water pressure sensors, particularly in marine surveys. Sometimes particle displacement sensors, particle acceleration sensors, or pressure gradient sensors are used in place of or in addition to particle velocity sensors. Particle velocity sensors and water pressure sensors are commonly know in the art as geophones and hydrophones, respectively. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, pressure sensors and particle velocity sensors may be deployed together in a marine survey, collocated in pairs or pairs of arrays along a seismic cable.
In a marine seismic survey, the seismic sensors are deployed in several conventional arrangements, including within ocean bottom cables laid on the water bottom, within vertical cables suspended in the water or in boreholes, or within substantially horizontal submerged cables (commonly called seismic streamers). The seismic streamers are typically towed through the water by ship, but alternatively may be maintained at a substantially stationary position, floating at a selected depth in the water.
The appropriate seismic sources for generating the seismic signal in land seismic surveys may include explosives or vibrators. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield may be of several types, including a small explosive charge, an electric spark or arc, a marine vibrator, and, typically, a gun. The seismic source gun may be a water gun, a vapor gun, and, most typically, an airgun. Typically, a marine seismic source consists not of a single source element, but of a spatially-distributed array of source elements. This arrangement is particularly true for airguns, currently the most common form of marine seismic source. Further, two or more source arrays are typically deployed in a marine seismic survey, to permit alternating activation of the source arrays.
Seismic sources and source arrays will be discussed in the context of arrays of airguns, since airguns are the most commonly-employed type of seismic source in the industry. A typical airgun contains a volume of air compressed to about 2000 psi (pounds per square inch) or more. Upon activation, commonly referred to as a “shot”, the airgun abruptly releases its compressed air to create an air bubble, leading to an expanding sound wave in the water. The resulting wave front constitutes the seismic signal that is reflected at the interfaces of subterranean earth formations and detected by the seismic sensors, as discussed above.
Although modern seismic sources such as airguns produce stable, repeatable wavefields in a laboratory situation, the wavefields produced by seismic source arrays deployed under natural conditions at sea are not so consistent. In a marine environment, the wavefields of seismic source arrays vary from shot to shot because the physical factors that determine the seismic source wavefields vary. If these seismic source variations could be monitored accurately, the seismic source variation data could be used to significantly enhance the quality of the resultant seismic data. Correcting for seismic source variations may be particularly important in situations requiring four-dimensional or time lapse seismic data acquisition, such as for reservoir monitoring. In these situations, the desired signal may comprise very small differences between acquired seismic data sets and this small signal may be obscured by noise introduced by the seismic source variations. However, shot to shot variations in marine seismic sources are often not monitored, primarily because it is difficult to do.
In the case of airguns, seismic source variations are caused by variations in the physical factors related to the individual airguns, the arrangement of airguns in arrays, and the environment. These physical factors include, but are not limited to, airgun depths, airgun pressure, airgun drop-outs (misfires), airgun timing, water surface conditions (roughness) affecting the ghost surface reflections, water temperature, local sound velocity in the water, atmospheric pressure, and three-dimensional geometry of the airgun arrays and sub-arrays. Some of these physical factors are measured during each shot. Other factors change more slowly and are thus measured less often.
The three-dimensional geometry factor includes the relative positions of all the elements of the airgun array. Since the airgun array may also comprise sub-arrays of airguns, the geometry includes the relative positions of airguns within sub-arrays as well as the relative positions of sub-arrays within the entire array. The array geometry determines how the individual air bubbles created by the airguns interact with each other. These bubble interactions have a significant effect on the generation of the source wavefield for the entire source array. Thus, variations in the three-dimensional geometry of the airgun arrays and sub-arrays have a significant effect on the variations in the seismic signals generated by the airgun arrays. If the airgun array is not rigid, then airgun geometry is typically measured for each shot. Relative positioning of the airguns sub-arrays is typically measured by GPS (Global Positioning System) receiver units mounted on the seismic source sub-arrays. Occasionally, however, the GPS receiver units fail. These failures lead to expensive downtime during the seismic surveys while the GPS units are repaired or replaced.
Thus, a need exists for a system for more accurate determination of the positions of source-array elements in a towed marine source array. This position-determination system could then provide an alternative or backup system for conventional means, such as GPS receiver units. Preferably, this position-determination system would also employ equipment already being used in current marine seismic surveys.